This invention relates to improved methods and apparatus for drilling holes in the earth utilizing a drill string containing an instrument which is connected by a flexible line to a read-out unit or other unit at the surface of the earth. The invention will be described primarily as applied to the drilling of generally horizontal holes into the earth, such as are frequently required for degasification of coal seams or the like, but it will be apparent as the description progresses that the invention is also applicable to the drilling of holes in other directions, such as generally vertically or at any selected inclination of slant.
In many drilling situations, it is desirable to monitor or control the drilling operation closely as it progresses, by an instrument which can sense a particular condition or attain a particular result only when the instrument is contained within the drill string. For example, an instrument may be employed which can sense the inclination of the drill string at a certain instant, and also sense the azimuth of that inclination, so that the drillers may then take such steps as are necessary to effect the correction required in order to finally reach a desired point in the earth formation. An instrument capable of functioning in this manner is shown in U.S. Pat. No. 3,791,043. The instrument probe of that patent may be contained in the drill string and connected by a flexible conductive line to a read-out unit at the surface of the earth on which the inclination and azimuth are indicated. An improved probe for such a system is shown in U.S. Pat. No. 3,862,499.
In using such inclination instruments, or other instruments which are connected to surface equipment by a flexible line, it has heretofore been necessary to remove the instrument and flexible line from the drill string each time that the length of the string is changed. This necessitates the expenditure of a substantial amount of time and effort on each such occasion, in first pulling the flexible line and instrument to the surface of the earth before addition or removal of a pipe section and then ultimately reinserting the line and instrument into the wall for further use.